Development and oceanographic applications of underwater in-situ radon sensor using plastic scintillator

Underwater in situ radon measurement is important scientific priority for oceanography, especially for survey and monitoring of submarine groundwater discharge (SDG). The high sensitivity and lightweight underwater in situ radon sensor using NaI(Tl) doped plastic scintillator was developed for oceanographic applications. A NaI(Tl) doped plastic scintillator can expect high sensitivity in comparison with a NaI(Tl) crystal sealed in a container because the plastic scintillator contacts seawater directly. The newly developed underwater in situ radon sensor can apply to various fields because the plastic scintillator is coated by light-resistant paint instead of using a dark chamber. Sensitivity sea tests and mapping survey were carried out at shallow hydrothermal area. The sensor was able to respond quickly to hydrothermal radon at seafloor and detect hydrothermal radon to 20 m above the hydrothermal point (seafloor). The sensor was carried along mapping grid (40 m × 40 m area, 5 m intervals) at 1 m layer above the seafloor by diver. The signals of the radon sensor ranged from 20 to 66 mV, and these signals corresponded with radon concentration of 2 to 12 becquerels per liter. Since the sensor is small and lightweight, measurement, monitoring and mapping can perform automatically by installing the sensor to an AUV (autonomous underwater vehicle). Furthermore, underwater in situ radon sensor is expected an application to earthquake prediction and volcanic activity monitoring as well as oceanography and hydrology.